THE PHYSICAL ENVIRONMENT OF ISLANDS 35to El Niño–Southern Oscillation (ENSO) phenom-
ena, can be of considerable ecological importance.
Islands may also experience periodic extremes such
as hurricanes (below) (Stoddart and Walsh 1992).
Contraction, or ‘telescoping’, of altitudinal zones
is another marked feature of smaller islands. For
example, on Krakatau, Indonesia, the plentiful
atmospheric moisture and the cooling influence of
the sea result in the near permanent presence of
cloud in the upper parts, further lowering tempera-
tures and allowing the development of a montane
mossy forest at around 600 m above sea level, a
much lower altitude than the continental norm
(Whittakeret al. 1989). Taylor (1957) suggested
2000 m as the height at which the vegetational tran-
sition occurs in interior Papua New Guinea, and
Whitmore (1984) observed that clouds normally
form around tropical peaks higher than 1200 m, but
that this altitudinal limit is reduced on offshore
islands. In his review of upper limits of forests on
tropical and warm-temperate oceanic islands,
Leuschner (1996) cited a range of 1000–2000 m for the
lowering of the forest line compared with that for
continental areas. Factors that have been invoked
for these reductions include steepened lapse rates
(and associated trends in atmospheric humidity
levels), droughts on trade-wind-exposed island
peaks with temperature inversions, the absence of
well-adapted high-altitude tree species on some
islands, and even the immaturity of volcanic soils
(Bruijnzeel et al. 1993; Leuschner 1996). Whatever
the precise cause(s) of the telescoping, it is a com-
mon feature of tropical islands, and it has the effect
of (potentially) increasing the number of species
that an island can support, by compressing habitats
and allowing a relatively low island to ‘sample’
additional upland species pools (Fig. 2.16).
The range of climatic conditions of an island is
determined in large measure by the elevation of the
highest mountain peaks. Thus regression studies
often find altitude to be an important variable in
explaining species numbers on islands, in some
cases ranking only second to, or even ahead of,
island area (Fernández-Palacios and Andersson
2000). Tenerife, in the Canaries, is an excellent exam-
ple of a volcanic island with a steep central ridge sys-
tem and a pronounced rain shadow. The Canaries lie
on the subsiding eastern side of the semi-permanent
Azores anticyclone at about 28N, 100 km off Africa.
The subsidence produces a warm, dry atmosphere
aloft that is separated at 1500–1800 m from a lower
layer of moist, southward-streaming air, producing
what is known as a trade-wind inversion
(Fernandopullé 1976). Tenerife’s climate is one of
warm, dry summers and mild, wet winters: in
essence, the island experiences a Mediterranean-
type climate, despite its latitudinal proximity to the
Sahara. The north-east trades bring in moisture from
the sea and the mountain backbone forces the air to
rise. It cools, and this produces a zone of orographic
clouds, locally known as mar de nubes, between about
600 and 1500 m, typically 300–500 m thick, on the
windward slope of the island—a feature that builds
up more or less daily (Fig. 2.17), providing the laurel
forest that grow at that altitude with the moisture
surplus (through fog drip and reduced evapotran-
spiration) needed to overcome the dry Canarian
summers (Fernández-Palacios 1992, Fig. 1). The3210Altitude (km)Approximate distance from coast (km)4.5 5 37 53Figure 2.16The transition to mossy forest on
mountains in Indonesia occurs at lower elevations
on small mountains near the sea than on larger
mountains inland. From left to right: Mt Tinggi
(Bawean), Mt Ranai (Natuna Island), Mt Salak
(West Java), and Mt Pangerango (West Java).
(After van Steenis 1972.)